Beyond applying brakes, engine compression braking is the most common method for reducing vehicle speed. For discrete ratio transmissions, fixed gear manual and automatic, this is accomplished by manually putting the transmission into a lower gear so that the engine turns faster. Because downshifting is a manual operation, the operator of the vehicle can select when and if they want to downshift.
With continuously variable transmissions (CVTs), such as toroidal transmissions, the downshift occurs automatically. CVT's are used almost universally in snowmobiles to alleviate any need for the driver to shift the transmission as the vehicle accelerates through its range of speeds. The assignee of the present invention has also successfully utilized CVT's in its line of ATV's. Typically the CVT is connected to the output shaft of the vehicle's engine, the transmission providing continuously variable gear reduction from the relatively higher rotation speed of the engine to the relatively lower rotation speed of the vehicle drive axle.
In some types of vehicle drive trains, when the vehicle is traveling along at a given speed and the throttle is dropped (e.g., to an idle speed), the rotation of the drive wheels of the vehicle will backdrive the drive train, causing the engine to rotate at a speed greater than it otherwise would based on throttle position. The inherent frictional forces present throughout the drive train, including particularly the compression forces present in the engine cylinders, tend to slow the vehicle down. This condition is commonly referred to as engine braking, and can be a useful feature. The degree of engine braking provided (in vehicles capable of doing so) is dependent on the gear ratio of the transmission, in higher gears less braking is provided, and in lower gears more braking is provided.
Unlike manual transmissions which allow an operator of the vehicle to downshift and thus increase engine braking, CVTs downshift automatically and thus do not allow the operator of the vehicle to choose how much and when the transmission ratio is changed. Some riders prefer more engine braking assist than others. The amount of engine braking desired often depends on the condition of the terrain where the vehicle is being used and on the application for which the vehicle is being used. When an ATV is operated in hilly terrain, engine braking is often desired in order to reduce brake effort and wear on the brake pads. When the vehicle is operated on flatter terrain, engine braking is often undesirable. In such conditions, riders often prefer to coast as far as possible. Riders that are sporty often want to maintain vehicle speed and they do not always want engine braking. Riders that are pulling trailers often do want engine braking to assist the hand brakes. Even when operated under the same conditions, for the same purpose, different riders have different preferences. The desired ratio change of the transmission is dependent on the terrain on which the vehicle is operating as well as rider preferences. Thus, it would be desirable to provide engine braking assist that is rider selectable to account for rider preferences and terrain conditions.